Of the disturbances of visual function, myopia and hypermetropia refer to the condition wherein the light that passed through cornea cannot form an image on the retina, and thus cannot grasp the image clearly. Of these, myopia is divided into axial myopia and refractive myopia according to the cause of the condition. The refractive myopia is caused by an increased refraction of cornea and lens, while axial myopia is caused by an elongation of the eyeball in the direction of optic axis, i.e. axial direction. It is nevertheless not easy to simply divide these two. The etiology of myopia has not been fully elucidated and a pharmaceutical agent to completely cure myopia has not been found yet.
In most cases, myopia is treated by a means utilizing correction of optical refraction. Correction of optical refraction by wearing glasses is not an ideal means as far as the quality of life and convenience for studying etc. are concerned. Correction of optical refraction using contact lenses often causes complications, and corneal ulcer may occur, which could possibly lead to the loss of sight in severe cases. In addition, recent application of corneal surgery to cure myopia is sometimes associated with failure to achieve expected levels of refraction, as well as occurrence of pain during operation and postoperative corneal opacity. In view of the fact that the correction of optical refraction and surgical operation such as the above-mentioned cannot be a perfect cure of myopia, treatment of myopia by the use of a drug is desired.
As the pharmaceutical agent to suppress axial elongation, the usefulness of muscarine I receptor antagonist and dopamine receptor agonist has been documented. However, a pharmaceutical agent which can be clinically applied has not been created so far.
Meanwhile, the retina consists of photoreceptor cells, bipolar cells, ganglion cells, horizontal cells, amacrine cells and the like, which transmit optical information to the central nerves. The functions of these cells contribute to the fulfillment of retinal function to organize the received optical information and transmit same to the central nerves.
When the retina is damaged, visual loss, disturbance of light sense and disturbance of visual field are induced, thereby causing central retinal artery and vein occlusions, congenital stationary night blindness, diabetic retinopathy, pigmentary retinal degeneration, retinal detachment, uveitis and the like.
While the therapeutics of retinal diseases and convalescence thereof vary depending on the kind and degree of the diseases, in particular, central retinal artery occlusion, diabetic retinopathy and retinal detachment scarcely allow complete recovery of retinal functions. It may happen that visual acuity does not improve after all and even an operation does not result in full recovery of visual acuity. What is more, no effective cure is currently available for pigmentary retinal degeneration but a symptomatic therapy such as use of sun glasses to avoid direct sun light.
In the internal treatment currently applied to cure retinal diseases, a pharmaceutical agent capable of suppressing degradation of retinal functions and achieving remarkable recovery of retinal function has not been found.
The asthenopia refers to a condition involving a kind of accommodation disorder of ciliary muscle due to systemic or local fatigue of the eye. Fatigue of eyes results in progressively growing distance of near point, which proceeds to the point that the eyes cannot recognize an object temporarily. However, recovery from fatigue can restore the original condition.
The treatment of asthenopia has heretofore included administration of medicaments such as vitamins (e.g., vitamin B.sub.1 and vitamin B.sub.12), ATP and the like, though sufficient therapeutic effects against asthenopia have not been attained.
As mentioned supra, no medicament that purportedly is useful for the prophylaxis and treatment of disturbance of visual function is satisfactory, and the development of an agent for the prophylaxis and treatment of disturbance of visual function has been desired by both doctors and patients.